This invention relates to electric switches and more particularly to snap action switches wherein a bridging contact is slidably driven into and out of bridging engagement with a pair of flexible stationary contacts.
U.S. Pat. No. 2,210,037 issued Aug. 6, 1940 to G. J. Meuer discloses a switch of the aforementioned type wherein the stationary contacts comprise resilient members supported on opposite end walls of the switch housing and having free ends overlying each other in spaced apart relationship within the central portion of the housing. A bridging contact is mounted on a insulating contact holder which is driven by an overcenter toggle mechanism for reciprocal sliding movement to carry the contact into and out of bridging engagement with the free ends of the stationary contacts. The contact holder is blanked, or sheared from a flat sheet of fiberboard and the bridging contact is a U-shaped member disposed within a lateral slot in the fiberboard contact holder to have the legs of the contact disposed horizontally along upper and lower surfaces of the contact holder. A rivet is provided through the fiberboard contact holder and the outer ends of the legs of the U-shaped bridging contact to maintain the contact assembled to the contact holder. Operation of the toggle mechanism drives the contact holder and bridging contact to a first position wherein the contact is disposed in bridging engagement with and between the upper and lower flexible stationary contacts to complete a circuit therebetween. Operation of the toggle mechanism to an opposite position drives the contact holder and bridging contact in an opposite direction to move the contact out of bridging engagement with the stationary contacts and to position a portion of the insulating contact holder between the free ends of the stationary contacts, thereby interrupting the circuit therebetween. Switches of this type have proven to perform very satisfactorily. However, the fiberboard contact holder is susceptible to swelling when the switch is used in humid, moist atmospheres, is susceptible to erosion from the electric arc that occurs upon contact separation, and can become brittle with very long life.
An improvement in the contact holder design is disclosed in U.S. Pat. No. 3,250,884 issued May 10, 1966 to R. E. Larkin. That contact holder is molded of an insulating phenolic resin and has U-shaped contacts which are assembled to the sides of the contact holder in lateral recesses as opposed to slots. The contacts have off-set legs which extend in the direction of motion of the contact holder to attain the desired length, and are recessed to be flush with the surface of the holder. The contacts are formed of good arc interrupting material and have a high quality current conducting material inlaid in the area of bridging engagement with the stationary contacts for enhanced performance. That contact holder is moisture resistant and less susceptible to arc erosion. Transversely disposed serrations are provided in that contact holder which engage the stationary contacts in the off position of the switch. Among other functions, these serrations provide a mild abraiding of the contacts for cleaning residue therefrom during movement of the holder. Testing and studies conducted on this switch suggest that the serrations may impart a vibration to the resilient stationary contacts during switch operation which may result in contact bounce and arcing upon bridging engagement of the contacts.
Switches of the aforementioned type are used extensively in industrial grade hand held portable electric tools. In some applications, particularly for tools manufactured in Europe, the separation distance between stationary contacts in the switch off position is required to be greater than presently exists in these switches. A direct solution to this requirement is to increase the thickness of the contact holder at the point where it engages the stationary contacts in the off position. The fiberboard contact holder such as in the aforementioned Meuer patent is not readily deformable to produce formations for this purpose, although such features can readily be incorporated on a molded contact holder such as in the aforementioned Larkin patent. In determining the nature of such formations, it has been discovered that particular configurations increase the circuit making and breaking performance of switches of this type.